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Common fates of photons

  1. Nov 23, 2012 #1
    Main fate of low energy photons in matter is photoelectric effect.

    At intermediate energies, it is Compton scattering (presumably mainly off electrons).

    At high energies, it is said to be "pair production".

    Pairs of what?

    At low energies, mainly electrons and positrons, obviously (neutrino pair production can be expected to have low intensity).

    But at higher energies?

    Is photonuclear effect a significant fate of photons compared to pair production, which remains possible at these energies?

    When the photon has sufficient energy, what is the branching ratio between p(γ,e+e-)p and p(γ,μ+μ-)p?
    And is a "pair" needed if it is bosons that are generated? For example, do conservation laws allow p(γ,π°)p? Or p(γ,π+)n?
  2. jcsd
  3. Nov 23, 2012 #2


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    Well, you can almost figure out the answer to this on a Post-It, just from dimensionality considerations. The cross-section for pair-production by a photon scattering off a nucleus is calculated to be σ(ω) = αZ2r02F(η) where α = e2/ħc is the fine structure constant, m is the mass of the electron, r0 = e2/mc2 is the classical electron radius, η is a dimensionless energy variable, η = ħω/mc2, and F(η) is a horrible function. Naturally F(η) goes to zero at threshhold, ħω = 2 mc2, or η = 2.

    That's pretty complicated, but wait -- what else could it be? σ must have dimensions of area, and the only length available is r0. So σ must go like 1/m2. For muon pair production at a corresponding energy, it's 1/(200)2 smaller.
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